The offset printing ink used in a sheet-fed offset press is known to dry slowly. Also, it is well known that the drying speed is considerably influenced by the temperature and humidity.
Therefore, the feed cylinder and deliver), cylinder on which printed papers are carried, are likely to be fouled with the ink from the printed papers, causing a trouble in the printing process.
Accordingly, it has been proposed to use an ink-fouling preventing sheet on the surfaces of a feed cylinder and delivery cylinder of an offset printing press to prevent the cylinders from being fouled with the printing ink oil printed papers. For example, the Japanese Unexamined Patent Publication No. 57-169360 discloses a protective sheet of this type made of a woven fabric having a surface support layer made of a low-friction material, such as fluoroplastic (applied with a repellant). This is shown in FIG. 1. The woven fabric 11 is attached to a metallic surface 10A of a skeleton wheel 10 having an appropriate fixture (such as Velcro or the like) attached on either end thereof to securely hold the woven fabric 11 on the wheel 10. However, such woven fabric is disadvantageous in its low durability and thus not usable for a long continuous printing.
Also, a similar sheet is disclosed in the Japanese Unexamined Patent Publication No. 53-7841. It is shown in FIG. 2. This is a sheet comprising a base 14 on which ball-like glass beads 12 are attached with an adhesive 13. It is attached to a feed or delivery cylinder to prevent the same from being fouled with the printing ink. This prior art is intended to assure point contact of the glass beads with printed papers to minimize the area of contact with the surface of the printed paper and the amount of friction with the printed paper. This prior art is effective in minimizing the transfer of the ink from printed papers.
However, it is difficult to continuously prevent the glass beads from being stained with the ink on printed papers only at the point contact of the glass beads with printed papers. As offset printing is continuously done for a certain length of time, the glass beads repeatedly touch printed papers and thus have a cumulated transfer of the ink from printed papers. The ink thus cumulated on the glass beads will stain the subsequently printed papers.
To avoid such fouling with the ink on printed papers, it is frequently necessary to stop the offset press and clean the glass beads with a solvent such as cleaning oil (petrolic cleaning solvent of a high boiling point) in order to remove the ink from the beads. Therefore, the press cannot be continuously operated for a long time. The time and labor required for the cleaning, those required for removing and cleaning the sheet or the those required for replacing the fouled sheet with a fresh one, share a considerably large part of the time and labor needed for a printing process.
Accordingly, the present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing an ink-fouling preventing sheet which can be used for a long time for daily printing for a long term without any cleaning with a cleaning solvent (a petrotic type of a high boiling point). The present invention provides for an offset printing press to be operated continuously for a long time of daily printing for a long term, and can be used as it is repeatedly washed in the cleaning oil after it is used for a long time, It has a highly improved durability.